From a user point of view, volume visualization allows the user to quickly inspect the volumetric image dataset and to select a region of interest to the user, such as a radiologist. The algorithms like combined intensity projection (CIP), maximum intensity projection (MIP), closest vessel projection (CVP), average intensity projection (AIP), and volume intensity projection (VIP) are based on ray casting. For each pixel of the displayed image, a ray is cast from said pixel through the image data volume. The intensity of a pixel is computed based on the intensities of voxels along the cast ray. For MIP, the intensity of the pixel is the intensity of a voxel along the cast ray having the highest intensity. For CVP, the intensity of the pixel is the intensity of a first voxel along the cast ray having a maximum intensity, possibly a local maximum intensity, greater than a CVP threshold. For AIP, the intensity of the pixel is the average of intensities of voxels along the cast ray. However, the information comprised in these images does not take into account the location of pixels along the cast ray.
In volume intensity projection (VIP), intensities of voxels along a ray cast from a pixel are transformed based on multiplied by weight factors. The larger the distance from a voxel to said pixel, the smaller the weight factor is. The voxels with weighted intensities are then used to compute an image, using e.g. MIP.
US 2010/0053159 A1 discloses a system for CIP rendering an image based on a volumetric image dataset, the system comprising: a computation unit for computing an initial pixel intensity of a pixel of the image, defined by a corresponding voxel intensity of a corresponding voxel comprised in the volumetric image dataset; and an adjustment unit for computing a final pixel intensity of the pixel, based on the initial pixel intensity and on a location of the corresponding voxel. Unlike a system using the volume intensity projection technique, which involves adjusting intensities of voxels comprised in an image dataset, the system of the cited application is arranged to adjust intensities of pixels. In particular, the final pixel intensity depends on a distance from the corresponding voxel to a viewing plane.